<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>最长的美好子字符串</title>
</head>
<body>
  <script>
    function fn(s) {
      const n = s.length;
      let maxPos = 0;
      let maxLen = 0;
      for (let i = 0; i < n; ++i) {
        let lower = 0;
        let upper = 0;
        for (let j = i; j < n; ++j) {
          if ('a' <= s[j] && s[j] <= 'z') {
            lower |= 1 << (s[j].charCodeAt() - 'a'.charCodeAt())
          } else {
            upper |= 1 << (s[j].charCodeAt() - 'A'.charCodeAt())
          }
          if (lower === upper && j - i + 1 > maxLen) {
            maxPos = i
            maxLen = j - i + 1
          }
        }
      }
      return s.slice(maxPos, maxPos + maxLen)
    }
    console.log(fn('YazaAay'))
  </script>
</body>
</html>